Thiophene-Based Conjugated Polymers with Photolabile Solubilizing

Feb 6, 2015 - Department of Physics and Astronomy and Center for Nanoscopic Physics, Tufts University, Medford, Massachusetts 02155, United. States...
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Thiophene-Based Conjugated Polymers with Photolabile Solubilizing Side Chains Zachary C. Smith,† Dianne M. Meyer,‡ Marc G. Simon,§ Cristian Staii,§ Deepak Shukla,‡ and Samuel W. Thomas, III*,† †

Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York 14650, United States § Department of Physics and Astronomy and Center for Nanoscopic Physics, Tufts University, Medford, Massachusetts 02155, United States ‡

S Supporting Information *

ABSTRACT: This paper describes a series of thiophene-based conjugated polymers that become insoluble upon irradiation with ultraviolet light. Stille or Suzuki reactions of appropriately substituted 2,5-bromothiophene derivatives yielded terthiophene and polythiophene derivatives with either o-nitrobenzyl (ONB) ester or ONB ether photolabile side chains with n-octyl substituents. Light-induced cleavage of these ONB side chains with ultraviolet light at 365 nm cleaves the octyl chains responsible for solubilization of the polymers away from the conjugated main chains, rendering them insoluble. Consistent with the accepted mechanism of ONB photolysis, those structural modifications that would yield a more stable benzylic radical methyl substitution on the benzylic position, replacement of the ester with an ether, or bothyielded more efficient photolyses as determined by (i) quantum yields of photolysis of ONB-substituted terthiophenes, and (ii) the percentage of polymer that persists in UV-irradiated thin films upon rinsing with chloroform. These polymers behave as negative-tone photoresists, enabling both direct photopatterning of conjugated polymers, and fabrication of multilayer conjugated polymer films by irradiating with UV light after each spin-casting step. Although hole-mobility values of these polymers in thin film transistors were only ∼10−5 cm2 V−1 s−1, photolysis and rinsing did not cause significant degradation in performance.

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INTRODUCTION This paper describes conjugated oligomers and polymers (CPs) with solubilizing chains connected to main chains through different photocleavable linkers. Conjugated polymers are active components of a number of solid-state optoelectronic technologies, such as sensors, transistors, and photovoltaics.1−3 Consistent among the vast majority of reported CPs are the presence of long alkyl chains, which impart solubility to these otherwise insoluble materials.4 Solubility of CPs is critical both during their preparationprecipitation during polymerization limits yield and molecular weight of polymersand for solventbased processing of materials into thin films, which is an important advantage that organic semiconductors have over their inorganic counterparts.5 Although state of the art semiconducting polymers generally possess them, solubilizing chains can present some drawbacks: (i) they restrict fabrication of solution-deposited multilayer films to materials that are orthogonally soluble,6 (ii) they occupy volume with optoelectronically inactive groups,7 and (iii) they participate in CP degradation processes, such as hydrogen atom abstractions.8 In a comprehensive study relating chemical structure of CPs to their photochemical stability, Krebs and co-workers recommended limiting their presence to “as low as possible”.7 © XXXX American Chemical Society

Several approaches exist for addressing the drawbacks of solubilizing chains. Cross-linking of polymer films is a popular choice for limiting solubility upon film deposition, although the alkyl chains remain in the film.9 There have been a number of examples of thermally cleavable solubilizing chains, focusing on tertiary esters that eliminate olefins and produce insoluble carboxylic acid-functionalized CPs.10,11 The resultant films can have performance that is superior to the films prior to thermolysis.10 More recently, acid catalysis for cleavage of solubilizing chains at lower temperatures has been reported, including of silane-substituted polythiophenes.12,13 Approaches that do not require high temperatures are important for eventual coating of CPs on inexpensive, flexible substrates such as poly(ethylene terephthalate).14 Photochemical approaches to changing the solubility of CPs have the advantage of spatiotemporal control and the possibility of mild conditions. Some approaches have included photo-oxidative cleavage of the CP backbone,15 photocleavage of solubilizing xanthate sideReceived: November 12, 2014 Revised: January 23, 2015

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Macromolecules chains,16 and photopatterning of a conjugated polymer using a photobase generator.17 The application of photocleavable groups to the design of light-responsive polymers and surfaces is a rapidly growing area of research.18,19 Although a number of photocleavable linkers exist, o-nitrobenzyl (ONB) groups are among the most popular, due to their ease of synthesis and the range of materials and experimental conditions in which they function.20,21 Pauly and Theato have used ONB groups in the photopatterning of conjugated poly(phenylacetylene)s.21 Our group has reported a polythiophene derivative comprising n-octyloxy chains connected to the conjugated poly(thiophene) backbone through photolabile ONB esters that cleave upon irradiation at 365 nm (Scheme 1).22 Upon irradiation, films of this polymer became

Chart 1. Rationale for Design of Photolabile Linkers: Increasing Radical Stability upon Intramolecular Hydrogen Atom Abstraction during Photocleavage

Scheme 1. o-Nitrobenzyl Ester Photocleavage of Previously Reported Polymer (P1) Resulting in the Analogous Carboxylic Acid Containing Polythiophene and the Nitroso Aldehyde Byproduct

of 1 with NaBH4 yielded primary nitrobenzyl alcohol 2, while reduction of 1 with CH3MgBr/TiCl4 gave the corresponding secondary nitrobenzyl alcohol 4. Acylation of either 2 or 4 with known compound 2,5-dibromothiophene-3-carboxylic acid24 gave the nitrobenzyl ester-functionalized dibromide-functionalized monomers 3 and 5. Stille cross-coupling of 3 with either 2-(tributylstannyl)thiophene or 2,5-bis(trimethylstannyl)thiophene yielded terthiophene O1 and polythiophene P1, while cross-coupling of 5 under similar reaction conditions yielded terthiophene O2 and polythiophene P2. The key intermediates to prepare analogous targets with nitrobenzyl ether linkers were 2,5-dibromothiophenes 7 and 9 (Scheme 3). Halo-dehydroxylation reactions of benzyl alcohols 2 and 4 yielded the corresponding benzyl bromides with PBr3 (6) or PPh3/CBr4 (8). Alkylation of 2,5-dibromo-3-(4hydroxyphenyl)thiophene with either of these alkyl bromides gave the nitrobenzyl ether-functionalized monomers 7 and 9. Stille cross-coupling of 7 with the appropriate stannanes yielded terthiophene O3 and polythiophene P3, while cross-coupling of 9 gave analogues O4 and P4, each of which has one methyl group in the benzylic position. Our synthesis of O4 by Suzuki coupling highlights an advantage of the ether linkage over the more commonly used esters, because the standard, strongly basic conditions for Suzuki coupling that we used can saponify esters.25 In addition, the use of Suzuki coupling reactions increases the number of easily accessible polymers because there are more commercially availableand less toxicdiboronic acids and esters than distannanes. As an example of this extended synthetic space, we synthesized the donor−acceptor polymers P5 and P6 using monomers 7 and 9, respectively, with the commercially available diboronic ester of benzothiadiazole (Scheme 4). Optical Properties of Conjugated Oligomers and Polymers. Tables 1 and 2 summarize the properties of absorbance and fluorescence emission of the terthiophenes and conjugated polymers studied here, while Figure 1 shows absorbance, emission, and excitation spectra of terthiophene O4. Absorbance spectra of the ONB-containing materials showed bands due to both the ONB chromophore in the UV region of the spectrum at approximately 285−320 nm, as well as the conjugated thiophene-containing main chains: terthio-

insoluble in toluene without significant photobleaching of the polythiophene backbone. The work reported herein summarizes detailed studies of this approach: (i) the effect of the chemical structure of the ONB group on the efficiency of photocleavage of solubilizing chains from terthiophenes and polythiophenes, (ii) the preparation of multilayer CP films using only one deposition solvent, (iii) the photopatterning of CP films, and (iv) hole mobility measurements in thin film transistors before and after photocleavage of solubilizing chains.

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EXPERIMENTAL DESIGN For the purposes of efficiency in processing, reducing the time required for photochemical cleavage of solubilizing chains is important. In addition, because both the CP backbones and photocleavage byproducts compete with ONB groups for absorption of light, reducing the number of photons required to render polymers insoluble also reduces photobleaching. We therefore designed conjugated oligomers and CPs with the photocleavable units shown in Chart 1. The key difference between these ONB groups is the stability of the corresponding benzylic radical photolysis intermediate. Our hypothesis was that those linkers with increased substitution or increased electron density at the benzylic position would cleave more efficiently: Bochet and co-workers recently showed a positive correlation between radical stabilization energy of ONB radicals and their quantum yields of photocleavage.23

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RESULTS AND DISCUSSION Synthesis of Conjugated Oligomers and Polymers. We prepared nitrobenzyl ester-linked terthiophenes and polythiophenes, which either lacked (O1 and P1) or contained (O2 and P2) benzylic methyl groups, as summarized in Scheme 2. The first steps in the synthesis of these targets was to define the substitution pattern at the benzylic position while reducing the aldehyde group of previously described compound 1; reduction B

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Macromolecules Scheme 2. Synthesis of Terthiophenes and Polythiophenes Containing n-Octyl Solubilizing Chains Bound through ONB Ester Photocleavable Linkers That Either Contain Either Zero (O1, P1) or One (O2, P2) Benzylic Methyl Groupa

Scheme 3. Synthesis of Terthiophenes and Polythiophenes Containing n-Octyl Solubilizing Chains Bound through ONB Ether Photocleavable Linkers That Either Contain Zero (O3, P3) or One (O4, P4) Benzylic Methyl Groupa

Key: (i) PBr3, 0 °C, 46%; (ii) K2CO3, DMF, 50 °C, 69%; (iii) Pd(PPh3)4, DMF, 90 °C, 43%; (iv) Pd2(dba)3, tri-o-tolylphosphine, toluene, 90 °C, 85%; (v) CBr4, PPh3, 52%; (vi) K2CO3, DMF, 50 °C, 84%; (vii) NaHCO3, Pd(PPh3)4, DMF/H2O, 80 °C, 82%; (viii) Pd2(dba)3, tri-o-tolyl phosphine, toluene, 90 °C, 92%. a

Scheme 4. Synthesis of Copolymers P5 and P6

a

Key: (i) NaBH4, MeOH, 70%; (ii) 2,5-dibromo-3-thiophenecarboxylic acid, DCC, DMAP, CH2Cl2, reflux, 43%; (iii) 2-(tributylstannyl)thiophene, Pd(PPh3)4, DMF, 90 °C, 83%; (iv) Pd2(dba)3,triotolylphosphine, 2,5-bis(trimethylstannyl) thiophene, toluene, 90 °C, 95%; (v) TiCl4, CH3MgBr, Et2O, −78 °C, 95%; (vi) 2,5-dibromo-3thiophene carboxylic acid, DCC, DMAP, CH2Cl2, reflux, 50%; (vii) Pd(PPh3)4, 2-(tributylstannyl)thiophene, DMF, 90 °C, 54%; (viii) Pd2(dba)3, tri-o-tolylphosphine, 2,5-bis(trimethylstannyl)thiophene, toluene, 90 °C, 82%.

phene chromophores absorbed in the UV between 320 and 350 nm, while polythiophenes P1−P4 have absorbance maxima between 510 and 530 nm, reflecting the extension of the conjugated π-systems upon comparing trimers to polymers. The absorbance and fluorescence spectra of the thiophene-cobenzothiadiazole polymers (P5-P6) are significantly red-shifted from those of the polythiophenes (P1−P4), which we attribute to their donor−acceptor character. The fluorescence quantum yields of those materials without ONB substituents have quantum yields of fluorescence similar to reported terthiophenes26 and polythiophenes;27 differences in molecular weight and regioregularity of polythiophenes also contribute to differences in optical properites. The nature of the interactions between chromophores is key to the study of any multichromophoric material. To help understand the interactions of the ONB and thiophene-based chromophores, we compared them to analogous compounds that did not contain ONB groups. Terthiophenes O5 and O6, which do not contain ONB groups, had 3−5 fold higher quantum yields of fluorescence than ONB-containing analogues

Chart 2. Chemical Structures of Oligomers O5 and O6

Table 1. Relevant Properties of Terthiophenes O1−O6 O1 O2 O3 O4 O5 O6

C

λmax(abs), nm

ε, M−1 cm−1

λmax(em), nm

ΦF

Φrxn

320 321 332 342 351 350

15000 17000 21000 17000 15000 19000

469 438 438 434 476 436

0.009 0.006 0.005 0.004 0.03 0.02

0.011 0.054 0.12 0.18 − −

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excitation of the terthiophene and not the ONB moiety. We therefore conclude that energy transfer from the ONB groups to the terthiophenes is not highly competitive with other photophysical or photochemical pathways, including bond cleavage, indicating the general applicability of using nitrobenzyl-based photocleavable groups in multichromophoric materials. ONB Photocleavage. As we have observed in previous studies of conjugated small molecules bound to nitrobenzyl groups,22,28 we found that fluorescence emission intensity from the terthiophene of O4 in solution increased upon irradiation with UV light, as shown in Figure 1. After irradiation at 365 nm for 3 min in CH2Cl2, the emission intensity of the sample increased 3-fold. This observation is consistent with photocleavage of the fluorescence quenching ONB groups, as upon their removal from the conjugated terthiophene backbone, the concentrations of any photolysis byproducts in solution are too small for diffusional quenching to compete with the unimolecular relaxation of the terthiophene excited states. One of our key objectives was to determine the relationship between the structure of the photocleavable ONB linkers and the efficiencies of their photocleavage reactions when bound to otherwise identical conjugated terthiophene backbones. We therefore monitored the progress of the photoreaction of each terthiophene oligomer by 1H NMR spectroscopy as a function of the number of photons absorbed from a 200 W Hg/Xe lamp at 365 nm, the flux of which we determined using the potassium ferrioxalate chemical actinometer. As the reactions proceeded, new resonances corresponding to each terthiophene photoproduct appeared with chemical shifts and multiplicities that matched the resonances observed for the expected terthiophene photoproducts, each of which we synthesized and characterized independently. Integrations of the resonances assigned to the starting material and product gave percent conversion. On the basis of our measurements of extinction coefficients of the terthiophene oligomers and model compounds designed to mimic the structures of the ONB pendants, the terthiophene chromophores absorb 87−90% of the incident light at 365 nm. Therefore, determination of quantum yields required accounting for the percentage of absorbed photons that the nitrobenzyl chromophore absorbed. As samples suitable for NMR spectroscopy were optically dense (OD > 3) at 365 nm, ratios of the extinction coefficients of the oligomeric chromophores and the ONB chromophores were sufficient to correct for the competitive absorbance due to the additive nature of the terthiophene and ONB extinction coefficients (see Supporting Information). These ratios, combined with the results of chemical actinometry, yielded the rate of photon absorbance of the photocleavable ONB pendants. Table 1 summarizes the quantum yields of photocleavage of the ONB groups of O1-O4. These results are consistent with our expectations detailed in the Experimental Design section: those structures that yield more stable benzylic radicals upon hydrogen atom abstraction have higher quantum yields of photocleavage than those that yield less stable benzylic radicals. The most dramatic example of this trend is the ONB photocleavage of compound O4, which has a methylated nitrobenzyl ether side chain: its photolysis was approximately 18-times more efficient than that of compound O1, which has an unmethylated nitrobenzyl ester. Photoinduced Film Insolubility. As most applications of conjugated polymers require that they be in the solid state, it is

Table 2. Characterization Details of P1−P7 P1 P2 P3 P4 P5 P6 P7

λmax(abs), nm

λmax(em), nm

ΦF

Mn, g/mol

PDI

520 509 529 520 546 544 464

627 590 626 579 676 663 581

0.14 0.19 0.10 0.13 0.13 0.17 0.33

18400 7000 12800 14800 46400 10200 6050

1.20 1.14 2.14 1.58 1.65 2.12 1.42

Figure 1. Top: Absorbance (solid lines), and emission (dashed lines) spectra for O4. Bottom: excitation spectra for O4. Black lines are the spectra of O4 in dichloromethane before irradiation; blue lines are the spectra of the same sample after 3 min of irradiation at 365 nm (40 mW/cm2). λex = 350 nm for emission spectra; λem = 450 nm for excitation spectra.

O1−O4; this observation is indicative of photoinduced electron transfer quenching of the fluorescence of the ONBcontaining fluorophores, as we have observed in previous studies of ONB-functionalized conjugated materials, and as is utilized in some photoactivatable dyes. Comparing ONBsubstituted polythiophenes P1−P4 to model polymer P7 showed a similar trend. In contrast to the evidence that the ONB group perturbs the excited state of the conjugated backbone by quenching fluorescence, the conjugated thiophene backbones have no more than a small effect on the excited states of ONB groups. As shown in Figure 1, there is a peak in the same range of absorbance of the nitrobenzyl group of O4 at 290 nm in the excitation spectrum. Photons absorbed in this range, however, contribute to only a small amount of terthiophene fluorescence, as the intensity of this peak in the excitation spectrum is heavily suppressed relative to its intensity in the absorbance spectrum. In addition, control oligomer O6, which has the same terthiophene chromophore, but lacks the ONB group, also has a band at 290−300 nm, indicating that the small peak at 290 nm in the excitation spectrum of O4 is mostly due to direct D

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intermediates. Figure 3 and Figure S3 shows comparisons that highlight this trend. Results from the thinnest films we prepared

Figure 3. Film retention of P2 and P4 as a function of film thickness under identical irradiation and washing conditions. Each film was irradiated for 4 min and then washed with CHCl3 for 30 s. Film retention was measured by monitoring the change in optical density in peak absorbance.

(thickness between 3 and 5 nm) showed that the largest difference of photoinduced insolubility between any two polymers was between P4 and P1: the solubility of P4, which has methylated ONB ether linkers, decreases significantly faster than that of P1, which has unmethylated ONB ester linkers, during UV irradiation (Figure S3). Thicker films showed an even larger difference in photoinduced film retention efficiency, as the comparison of films of P4 and P2 in Figure 3 shows: after 4 min of irradiation followed by rinsing in CHCl3 3−5 fold more of P4 is retained. Donor−acceptor polymers P5 and P6, which have ONB ether linkers, also showed photoinduced insolubility, with 90% retained of a 27 nm thick film of P6 after irradiation at 365 nm for 20 min and rinsing with CHCl3 (Figure 4). We attribute the hypsochromic shift of the polymer

Figure 2. NMR spectra from the photolysis of (a) O1 and (b) O3 in CDCl3. Faint vertical lines indicate resonances attributed solely to the terthiophene photoproduct, the integrals of which we used to determine conversion.

important to understand how structure of the photocleavable groups affects the photoinduced resistance to dissolution of polymer films. Photobleaching, which is often observed upon exposing organic chromophores to light and air simultaneously, is a primary concern in the design of our materials and the execution of these experiments. Addressing this potential problem is an important reason that we focused on thiophene-based polymers, which are known to be relatively resistant to photooxidation.29 Thin films of our polymers were resistant to photobleaching upon irradiation with UV light under ambient conditions: Figure S2 shows that 1−2 nm thick films of P3 showed less than 7% bleaching of its main absorbance band at 520 nm upon irradiation at 365 nm (40 mW/cm2, 10 min). All polymers described here displayed similar robustness with respect to photobleaching, which we observed during film retention experiments (vide inf ra). Irradiating an identical film of P3 in an argon atmosphere prevented even this small amount of photobleaching. Having established the photostability of our materials, we investigated how photocleavable linker structure impacted the efficiency of rendering polythiophene films insoluble with UV irradiation. To do so, we irradiated spun-cast thin films of polymers P1−P4 on glass substrates at 365 nm, followed by rinsing in a bath of CHCl3 for 30 s. Comparisons of UV/vis absorbance spectra of films before and after irradiation yielded the degree of photobleaching, while comparisons of spectra before and after rinsing the irradiated films yielded the percent retention of the polymer backbone on the substrate. In all cases, unirradiated spun-cast films were freely soluble in CHCl3. In agreement with the quantum yields of photocleavage we measured in solution, as well as our expectations based on our experimental design, the solubility of films of polymers with more stable benzylic radical intermediates were more sensitive to UV irradiation than polymers with less stable radical

Figure 4. Absorbance spectra of a film of P6 before irradiation (black line), after 20 min of irradiation at 365 nm (orange line), and after washing with chloroform for 30 s (blue dashed line).

upon rinsing to a decreased density of conjugated polymer in the film combination of swelling with solvent and dissolution of shorter conjugated polymer chains. In contrast to the retention of material as determined by UV/vis spectrophotometry, film thicknesses as determined by AFM decreased significantly upon irradiation and rinsing. Spun cast films of P4 that were initially 33 ± 3.0 nm decreased to 23 ± 1.5 nm after irradiation for 20 min. After rinsing of the irradiated films, the film thickness decreased to 16 ± 3.0 nm, E

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Macromolecules even though absorbance decreased by only ∼10%. Preliminarily, we attribute these changes to (i) annealing and closer packing of the polymer chains during UV irradiation, and (ii) removal of quantities of both small molecule photocleavage products and shorter polymer chains that remained soluble even after photolysis (vide inf ra). This explanation is consistent with the consistent red-shifting of absorbance spectra of these polymer films after irradiation but before rinsing (Figure 4). Our experiments showed that even after prolonged irradiation times (up to 40 min), less than 100% of the polymer film remained after rinsing. UV/vis spectroscopy of the CHCl3 rinsing solution for a film of polymer P6 that had been irradiated at 365 nm for 20 min shows a peak at 325 nm that is due, at least in part, to the o-nitrobenzyl side-chains, and a broad peak with maximum at 565 nm corresponding to the poly(thiophene-co-benzothiadiazole) backbone. This peak in the rinsing solution, however, is shifted hypsochromically from the as-synthesized P6 in CHCl3 by ∼16 nm (Figure 5). In

Figure 6. Optical microscopy images of a photomask (A and B) and images of films of P4 photopatterned using the photomask (C and D).

group of moderately polar organic solvents. This shared characteristic presents a challenge for using solution-based processes to fabricate films with multiple layers, which organic photovoltaics (OPVs) and organic light emitting devices (OLEDs) often require. The capability to render a solutioncast conjugated polymer film insoluble in a postdeposition processing step, such as with photochemical cross-linking,30 or photochemical acid generation leading to a change in polymer solubility,31 can allow for the preparation of multilayer films from solution while only minimally disturbing previously deposited layers. Figure 7 illustrates the capability of our approach to yield multilayer films with all solution-based processing. We first performed multiple spin-casting steps of a solution of P6 in CHCl3 onto a glass substrate without any irradiation in between deposition steps. The absorbance of the film did not increase significantly upon spin-casting additional polymer after deposition of the initial film, as the solvent used during each spin-casting step dissolved the previously deposited film. We irradiated the film with 365 nm light (34 mW/cm2) for 10 min after the fourth deposition step, which resulted in a decrease of intensity and bathochromic shift of the UV-absorbing band, consistent with cleavage of the ONB groups, but no significant change in the band in the visible region of the spectrum, which indicated minimal photobleaching of the conjugated poly(thiophene-co-benzothiadiazole) backbone. A subsequent spincasting step of P6 under identical conditions as used in the first four deposition steps increased the absorbance of the film. Rinsing that film with CHCl3 confirmed that this increase in thickness was due to light-induced insolubility of the irradiated film, as the absorbance spectrum of the resultant, thinner film closely resembled that observed after irradiation of the previously deposited film. We could repeat this process multiple times with either the same polymer (P6), which has potential utility in the fabrication of thicker films with polymers that have limited solubility, or switch between photoreactive polymers that have different optoelectronic properties between deposition/irradiation steps (alternating layers of P4 and P6). Fabrication and Evaluation of Organic Thin Film Transistor Devices. To probe carrier transport properties, ptype thin film transistor devices incorporating polymers P1, P2, P5, and P6 were made using the bottom-gate, top-contact

Figure 5. Left: The normalized absorbance spectra of P6 in CHCl3 (black) and the rinsing solution from an irradiated film of P6 (blue). Right: The normalized GPC chromatogram for P6 (black) and its corresponding rinsing solution (blue).

addition, GPC analysis of the rinsing solution shows low molecular weight polymer chains and small molecule byproducts of photolysis, with less contribution from higher molecular weight chains of the as-synthesized sample of P6 used to prepare the films. This evidence indicates that some shorter polymer chains remain soluble, even upon ONB photolysis, and their dissolution is a primary cause for the 10− 20% decrease in absorbance upon rinsing of irradiated films. Photopatterning of Conjugated Polymer Films. The patterning of electronically active materials, using photolithography as a key step, is a cornerstone technology of the electronics industry. Conjugated polymers that show lightsensitive solubility open the possibility of semiconducting organic materials that can form intricate patterns easily and directly with minimal additional processing steps.15,16 Our approach of photocleavage of solubilizing chains enables conjugated polymers to be used as negative photoresists. Figure 6 shows typical results of a photopatterning experiment with polymer P4. Irradiation of a 23 nm thick of P4 at 365 nm (34 mW/cm2) for 20 min through a chrome photomask followed by rinsing with CHCl3 for 30 s yielded a pattern of conjugated polymer: those areas of polymer film exposed to light remained on the substrate, while those polymers in areas shielded by the mask dissolved. Although imperfections such as rounded corners indicate room for improvement of photolithographic conditions, our nonoptimized patterning procedure could resolve features on the order of 10 μm. Multilayer Films Enabled By Photoinduced Insolubility. Most reported conjugated polymers are soluble in the same F

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Figure 7. Multilayer deposition experiments: Left: Thin film multilayer experiment without irradiation until after spinning P6 onto the substrate four times. Middle: Thin film multilayer of P6 made by sequentially spin coating and irradiating layers for the first three deposition steps. The fourth layer was deposited and then washed away without irradiation, demonstrating the necessity of UV irradiation to retain the film on the substrate. Right: Multilayer film made from alternating deposition of P4 (blue lines) and P6 (black lines). Each layer was spun on top of the previous layer after a 10 min irradiation at 365 nm. After the final layer was deposited, it was not irradiated and the layer was washed away with chloroform to give the final spectrum (black dashed line).

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geometry, in which heavily doped silicon (resistivity 0.005− 0.025 Ω-cm) substrate served as the gate electrode. The gate dielectric was a thermally grown SiO2 layer with a thickness of 190 nm modified with hexamethyldisilazane (HMDS). P1, P5, and P6 showed typical p-type field effect operation with excellent output and transfer characteristics. Representative output characteristics (drain current (IDS) versus source-drain voltage (VDS) curves at different gate voltages (VGS)) and transfer characteristics (ISD vs VGS plotted on a logarithmic scale and (ISD)1/2 vs VGS, at VDS = 100 V) of the transistors based on P1 at 25 °C are shown in Figure S5. For a device with 150 μm channel length (L) and 1000 μm channel width (W), a saturation regime hole mobility of 2.6 (±0.05) × 10−5 cm2 V−1 s−1 and Vth of 52 V and current on/off ratio of 4.5 × 103 was extracted for VDS = −100 V. These devices exhibit low contact resistance and as a consequence saturation mobility does not decrease significantly with decreasing channel length. The electrical performance of polymers P5 and P6 were similar to P1 (Table S1). Polymer P2 did not show any measurable hole mobility in this device architecture, to which impurities in the polymer sample may be a contributing factor. We also compared electrical performance of our polymers with commercially available regioregular P3HT (Aldrich), which showed higher carrier mobility (ca. 10−3 cm2 V−1 s−1) than our polymers. Preliminarily, we attribute these differences in mobility in part to higher crystallinity of P3HT, due to the lack of regioregularity and relatively low molecular weights of polymers described in this paper. Importantly, the electrical performance of P1 and P6 did not degrade significantly after the polymers were exposed to light and rinsed with chloroform (Figure S5, Table S1). For example, OTFT devices made with P1 after exposure to light and rinsing with chloroform did not show any significant change in electrical performance or drop in carrier mobility. Thus, for devices with W = 1000 μm and L = 100 μm, saturation mobility μ = 1.6 (±0.05) × 10−5 cm2 V−1 s−1 was only slightly lower than for devices before irradiation and rinsing (Figure S5).

CONCLUSION

This work demonstrates that photocleavable solubilizing side chains can give conjugated polymers the properties of negative photoresists, enabling photopatterning and the fabrication of multilayer films using solution processing. Our observations that photolysis of ONB groups need not cause significant photobleaching or complete disruption of the charge transport properties of these materials supports the potential applicability of this approach. Photocleavable side chains generally have the advantages that they neither require the addition of additional compounds in formulations such as photoacid or photobase generators, nor do they depend on degradation of the conjugated backbone. The ONB photocleavable moiety has a number of specific advantages in this context: (i) their photolysis efficiencies are not affected significantly by potential energy transfer deactivation pathways to the conjugated material, (ii) they have tunable sensitivity to light through controlling the stabilization of the intermediate benzylic radicals, (iii) their functionalization and installation is straightforward, and (iv) they do not require highly hydrophilic environments to cleave efficiently. Primary impediments to the eventual success of this approach include the presence of potentially deleterious photolysis byproducts, and the creation of voids within films. Questions this work has not addressed are the potential role of minor extents of photoinduced crosslinking of polymer chains in the thin films due to radical intermediates of ONB photolysis, or the sharpness of the interfaces between different polymers in multilayer films, but we anticipate that the degree of interpenetration of polymers may be tunable based on the extent of photolysis. Current work in our laboratory is focused on expanding the classes of monomers that incorporate photocleavable solubilizing chains, as well as the development of conjugated polymers that behave as positive-tone photoresists, which could overcome the challenges described here. G

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ASSOCIATED CONTENT

S Supporting Information *

General considerations, all experimental procedures, including procedures for syntheses of new compounds, NMR spectra, and additional absorbance and fluorescence spectra, transistor and I−V characteristics, AFM images, and quantum yield calcualtions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(S.W.T.) Telephone: 617-627-3771. E-mail: sam.thomas@ tufts.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support for this work was provided by the National Science Foundation (CAREER Award DMR-1151385 and CBET-1067093).

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REFERENCES

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DOI: 10.1021/ma502289n Macromolecules XXXX, XXX, XXX−XXX